Interesting idea, having two cameras motion needed for trigger. As long as the cameras were set physically in such a way to be sure to read motion from the intruder, it could work. Unfortunately, there’s no direct way to do that with IFTTT. I know it’s possible in IFTTT to have multiple actions follow a single trigger. I’m not aware of any way to require multiple triggers to cause an action. However, I’m not what you’d call an IFTTT expert.
There may be some clever way to string together recipes to make this happen. Something like having the first camera’s motion trigger an intermediary action that somehow sets a flag. Then when the second camera’s motion is triggered, the siren is triggered only if the flag is set. I have no idea what the intermediary would be though.